In spark erosive cutting, workpieces are worked by means of a tool electrode cutting wire under the control of a digital tool control system on the basis of predetermined cutting path information. The cutting wire and workpiece are connected to a generator which, under the influence of a control circuit, supplies clearly defined current or voltage pulses for maintaining the spark erosive process between the cutting wire and the workpiece. The shapes to be cut in the workpiece frequently form a closed path curve, so that the cutting wire returns to its initial position towards the end of the cutting process, forming a fall-off piece which threatens to become detached from the fixed workpiece. In accordance with the user's requirements, either the cut out inner part or the surrounding outer shape is intended for further use. There is now a danger towards the end of the cutting process of an unsecured fall-off piece destroying the wire electode or sticking to parts of the working device on becoming detached from the remaining fixed workpiece. It is not desirable to also fix the fall-off piece when cutting closed curves, because in this case free access to the cutting path by the wire guidance heads would not be ensured over the entire cutting path, so that resetting of the workpiece would be necessary. In view of the use of the device in mass production operations and the desire for maximum automation of operations, this would be undesirable.
Measures are already known for securing a fall-off piece. For example, a thin residual web between the workpiece and the fall-off piece may be left after cutting to secure the fall-off piece, the latter being eventually detached from the remaining workpiece by an external force application, such as a hammer blow at the end of the cutting process. The disadvantage of this process is that a manual operation is necessary, which should be avoided with the aim of achieving automatic machining methods. Another disadvantage of the known process is that the web must be carefully dimensioned with respect to the material to be cut, so that the fall-off piece can in fact be knocked off. Another disadvantage is that the breaking point generally does not permit any subsequent machining, so that the clean working of closed cutting curves is not satisfactorily ensured.
In view of these difficulties, it has already been proposed (JP-OS No. 56-126,526/1981) as an alternative to the conventional practice to arrange the cutting wire in a horizontal manner and to fix the workpiece in such a way that, due to the desired cutting curve, a cut off inner part sinks vertically downwards and is supported on the already cut surface of the workpiece. Such a process can only be used if there are relatively wide areas of cut, i.e. large workpiece thicknesses are available, and if the cutting track is not too wide compared with the width of the cut contour. Therefore, general use of this process is not possible.
A further proposal (JP-As 51-116,674/1976) is directed at an adhesive-bonded joint between the fall-off piece and the remainder of the workpiece. A mechanical and electrical connection between the two parts is produced with the aid of an adhesive or with the aid of holders or clips. However, this process cannot be used in a fully automated working operation. According to another proposal (DE-OS No. 2,351,357), metals with a low melting point are heated and dripped into the already cut path, or the latter is bridged with the aid of anaerobic adhesives. The disadvantage of this proposal is that the corresponding dosing or metering means must be aligned very accurately with the already cut and usually very narrow track, in order to ensure the penetration of the bonding medium. The joint which is formed between the fall-off piece and the workpiece obstructs a trimming or shaving operation, in which the cutting mechanism is used automatically, without resetting the workpiece, for improving the surface quality of the cut workpiece or for the fine trimming of the corners. It is also a disadvantage that at the end of the cutting process, bonding medium residues are left behind on the worked surfaces and have to be laboriously removed therefrom.
The use of controllable, movable mechanical holding means for receiving the fall-off piece has also proved unsatisfactory, because such means often exert a disturbing force on the wire electrode, which generally leads to cutting imperfections on the workpiece and, due to the additional mechanical work, causes difficulties when installing the machine and during the actual cutting process.